问题
This is one of those "pretty sure we found the answer, but hoping we're wrong" questions. We are looking at a steganography problem and it's not pretty.
Situation: We have a series of images. We want to mark them (watermark) so the watermarks survive a series of conditions. The kicker is, we are using a lossfull format, JPG, rather than lossless such as PNG. Our watermarks need to survive screenshotting and, furthermore, need to be invisible to the naked eye. Finally, they need contain at least 32 bytes of data (we expect them to be repeating patterns across an image, of course). Due to the above, we need to hide the information in the pixels themselves. I am trying a Least Significant Bit change, including using large blocks per "bit" (I tried both increments of 16 as these are the jpg compression algorithms size chunks from what we understand, as well as various prime numbers) and reading the average of the resulting block. This sort of leads to requirements:
- Must be .jpg
- Must survive the jpg compression algorithm
- Must survive screenshotting (assume screenshots are saved losslessly)
Problem: JPG compression, even 100% "minimum loss" changes the pixel values. EG if we draw a huge band across an image setting the Red channel to 255 in a block 64 pixels high, more than half are not 255 in the compiled image. This means that even using an average of the blocks yields the LSB to be random, rather than what we "encoded". Our current prototype can take a random image, compress the message into a bit-endoded string and convert it to a XbyX array which is then superimposed on the image using the LSB of one of the three color-channels. This works and is detectable while it remains a BufferedImage, but once we convert to a JPG the compression destroys the message.
Question: Is there a way to better control a jpg compression's pixel values? Or are we simply SOOL here and need to drop this avenue, either shifting to PNG output (unlikely) or need to understand the JPG compression's algorithm at length and use it to somehow determine LSB pattern outcomes? Preferably java, but we are open to looking at alternative languages if there are any that can solve our problem (our current PoC is in java)
来源：https://stackoverflow.com/questions/59880267/maintaining-lsb-through-jpg-compilation-is-it-possible